The Catalytic Conversion of Methyl Chloride to Ethylene and Propylene over Phosphorus-Modified Mg-ZSM-5 Zeolites

A Mg-ZSM-5 zeolite modified with phosphorus is capable of catalyzing the reaction of CH3Cl to C2H4, C3H6, C4H8, and HCl at 500°C. At a WHSV of 20 h−1, an initial conversion level of 96% was achieved with combined C2H4, C3H6, and C4H8 selectivities of about 80%. During the useful life of the catalyst the C3H6 selectivity was 50-60%. The percent conversion decreased to 50% over a period of 20 h, but the catalyst could be regenerated by heating in flowing air. As the catalyst deactivated, the C3H6 selectivity increased slightly and the C2H4 selectivity decreased. Catalytic and spectroscopic results confirm that phosphorus, derived from trimethylphosphine, was responsible for a decrease in the strong Brønsted acidity in the zeolite. For example, the phosphorus-modified zeolite was inactive for n-hexane cracking at 350°C, and the protonated amount of pyridine, added to the zeolite as a probe for acidity, decreased significantly. The catalyst, however, had sufficient acidity to crack hexene or octene at 500°C to propylene and ethylene in ratios that were very similar to those detected during the conversion of CH3Cl. Without the strong Brønsted acidity the PMg-ZSM-5 zeolite apparently is unable to convert the light olefins to paraffins and aromatics. A mechanism is proposed in which magnesium cations activate CH3Cl to form HCl and a carbene intermediate. The latter is believed to be responsible for CC bond formation via reaction with a surface methoxide species. Ethylene probably is the primary hydrocarbon, but it oligomerizes to a higher molecular weight olefin which cracks back to ethylene and propylene.